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Saturday, July 25, 2009

Alternative Approach to Measuring Venus Surface Composition

From Bruce Moomaw:

There are two important shared components in both the two Venus Landers proposed by NASA's Venus Science and Technology Definition Team ( http://www.lpi.usra.edu/vexag/reports/venusFlagshipMissionStudy090501.pdf ) and the lighter and cheaper landers in the "SAGE" New Frontiers proposal made by a team led by the Univ. of Colorado's Larry Esposito ( http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38184/1/03-2520.pdf ). Those components are: (1) an X-ray diffractometer/spectrometer to analyze Venusian surface minerals for the first time, and (2) a complicated sample-collection system involving a drill that will bore up samples from 10 cm below the surface and a little airlock to transfer the samples into the room-temperature interior of the armored and insulated lander so that the X-ray instruments can analyze them without our having to use prohibitively expensive revolutionary high-temperature electronics.

That drill/airlock system -- although the Soviet Union used it for X-ray spectral analysis of elements in the Venusian surface on three landers -- is complex, heavy and power-consuming. In Esposito's SAGE landers, it weighs 26 kg and uses 90 watts of power; in the STDT landers, it weighs 35 kg and uses 120 watts. It's also complex enough to be vulnerable to failure (and in fact it did fail on three of the six landers the Soviets equipped with it) -- and if it develops a leak, it could wreck the entire mission by letting Venus' super-hot and high-pressure atmosphere into the lander's interior. Moreover, the X-ray instruments require lengthy sample observation time to get a proper analysis of the less common but scientifically important elements in the sample -- the SAGE lander would spend two hours obtaining adequate X-ray spectra on a single sample, while the STDT landers would take 2.5 hours to analyze each of two surface samples. This naturally requires that the landers be sufficiently armored against Venus's savage surface conditions to last that long, which in turn greatly increases their overall mass and cost.

Is there an alternative? It appears that there is. The Mars Surface Laboratory carries an instrument called "ChemCam", which will actually use a miniature pointable laser to briefly heat points on the Martian surface to incandescence and take visual spectra of the glow to identify even quite scarce elements in just a few seconds. This "laser-induced breakdown spectrometry" ("LIBS") to determine element percentages can also be combined with Raman spectrometry, which makes use of the fact that a small fraction of the laser's light is reflected back at modified frequencies due to the light's interaction with various minerals -- and which, once again, can make its measurements very rapidly. The ChemCam on MSL doesn't have Raman capability, but such a combined instrument is scheduled for the ESA's ExoMars lander.

Preliminary lab tests indicate that combined LIBS/Raman spectrometry will also be completely feasible on Venus, despite the dense and hot CO2 atmosphere that the laser pulse and its reflection must travel through:

Such an instrument would completely remove any need for a drill, an airlock, and long sample analysis times -- it could operate by peering through the same windows in the Lander's hull used by its cameras and infrared spectrometers, analyzing patches of surface up to several dozen meters from the lander. It would use much less power than a drill. And the speed of its operation would allow analysis of a dozen or more different spots even if the lander survived only a half-hour or so on the surface.

How comprehensive would a LIBS/Raman analysis be? LIBS can definitely detect all the same elements as X-ray fluorescence spectrometry. And Alian Wang of Washington University tells me that Raman spectrometry, in fact, can unambiguously identify almost all of the same minerals as an X-ray diffractometer except for halide salts, which are very unlikely to be found on Venus. In particular, it can identify amphiboles and other minerals which could be a giveaway to the existence of an ancient liquid-water ocean in Venus' earliest days. Even if Raman spectrometry would miss a few of the minerals detectable by the X-ray technique, the advantages of LIBS/Raman spectrometry -- both in making the lander simpler and cheaper, and in analyzing multiple different places on the Venusian surface -- provide a very strong argument for using it rather than the Rube Goldbergian drill/airlock/long-lived X-ray system. For these reasons the Jet Propulsion Laboratory's 2007 "Planetary Science Summer School" of graduate student interns chose it for the minimum-cost "VEIL" Venus lander they were assigned to design ( http://www.lpi.usra.edu/vexag/nov_2007/presentations/schmidt.pdf ) -- although their version would analyze only one spot on the surface.

One possible problem might be that the chemical reactions of Venus' hot atmosphere and its trace gases with surface minerals might produce a surface rind whose composition would be misleading when it comes to identifying the original minerals in Venus' surface rock. But it might be possible -- as the PSSS interns suggest -- to equip the lander with a grinding wheel on a swivelable arm, such as the two MER rovers have used to scour away coatings on Mars rocks, allowing the lander to analyze virgin rock in at least a few nearby places -- a setup which would still be much simpler and more reliable than a full-fledged coring drill on the lander. This is not to mention the fact that the LIBS pulses, if fired repeatedly at the same location, might be able to do some surface scouring themselves by vaporizing a thin surface coating away.

At any rate, this looks like a highly promising new technique to study a particularly hostile and difficult planet with a minimum of cost combined with a lot of scientific return.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.